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Other humans merit moral concern. We think many nonhumans merit moral concern too. But how do we know? And which nonhumans? Chimpanzees? Chickens? Bumblebees? Protozoa? Roombas? Rocks? Where and how do we draw a line?

What would it take to justifiably believe that some nonhuman experiences pain (or pleasure) in a morally significant way?1 This is a tough question, but it is incredibly important to get right. Humans constitute a very, verysmall fraction of the animal kingdom. If other vertebrate animals experience morally significant pain, then much of our engagement with these animals is deeply immoral. If invertebrate animals experience morally significant pain, then, given the sheer number of invertebrates,2 an almost incomprehensible amount of morally significant suffering occurs beyond the ken of normal human attention. And if the capacity to experience morally significant pain is not restricted to organic entities, then human civilizations of the future may be capable of producing exponentially more sentient entities than presently exist.

On the other hand, if many, most, or all nonhumans do not experience morally significant pain, then it could be a waste of resources to try to change their condition. Given that there are millions of humans currently experiencing morally significant pain (for whom these resources would be a great aid), the opportunity cost of wasting time, talent, and money on nonhumans appears tremendous.

Figuring out where and whether to allocate resources to help nonhumans is of significant interest to Rethink Priorities. This post is our first in a series on morally significant pain in invertebrates.